Modern communications systems (and satellite communications systems in particular) commonly provide bandwidth to a region of interest by dividing one or more wide bandwidth downlink beams into multiple lower bandwidth frequency components. For example, spot beams transmitted by a satellite may be assigned one of four possible frequency bands carved out of a single wide bandwidth downlink beam. The spot beams of each frequency band are assigned ideally non-overlapping and non-interfering coverage areas over the region of interest. In other words, the four frequency bands are part of a frequency reuse pattern designed to cover the region of interest.
A frequency reuse plan using four distinct frequencies is commonly referred to as a four-to-one frequency reuse pattern. A frequency reuse scheme, however, may also be implemented as three-to-one, seven-to-one, or twenty-one-to-one frequency reuse pattern. In general, frequency reuse patterns may be implemented as (i.sup.2 +ij+j.sup.2):1 for any integer i,j. A frequency reuse pattern physically separates spot beams of a particular frequency band from each other using spot beams assigned to other frequency bands. For example, in a four-to-one frequency reuse pattern, a spot beam of a particular frequency band covers a cell (a portion of the region of interest). Additional spot beams then cover the six surrounding cells in an alternating scheme that guarantees that no nearest spot beam neighbors use the same frequency band. Frequency reuse patterns, however, still suffer from co-channel and adjacent channel interference (collectively "interference") which may limit the total bandwidth capacity of each frequency band.
Co-channel interference is interference generated in a spot beam assigned to a particular frequency band by nearby spot beams assigned to the same frequency band. Co-channel interference occurs even though spot beams assigned to a particular frequency band are physically separated. In part, the amount of co-channel interference depends on the number of nearby spot beams covering the same frequency band because real antennas cannot completely isolate a spot beam in a region of interest (for example, a cell).
Adjacent channel interference is interference generated in a spot beam assigned to a particular frequency band by neighboring spot beams of other frequencies. One common cause of adjacent channel interference is imperfections in the antennas used to generate the spot beams. Because virtually all antennas generate sidelobes, the energy transmitted in each spot beam is not perfectly confined to their assigned coverage area. Additionally, all real modulation techniques generate frequency sidelobes which overlap with the frequency bands of neighboring cells. As a result, spot beams may spill over in frequency into neighboring spot beams and cause adjacent channel interference.
The energy spillover effect described above is, in fact, common to both types of interference. The difference, however, is in the location of the nearby spot beams and the frequency band over which the spot beam operates. The cause of interference is the combination of the antenna effect described above and the fact that any modulation generates frequency sidelobes that are at the same frequency as the frequency band in the spot beam of interest. The effect of the antenna sidelobes is to reduce that interference by the amount of the gain of the antenna in the direction of the cell of interest.
In the past, attempts to minimize co-channel and adjacent channel interference have included expensive, complicated, and sophisticated antenna designs that minimize sidelobes and complex modulations that minimize frequency sidelobes. Even with the added complexity and expense of more sophisticated antennas, sidelobes cannot be completely eliminated. As a result, the communications system becomes much more expensive yet co-channel interference cannot be completely eliminated.
Other attempts at minimizing adjacent channel interference have included using guard bands or filters around the spot beams assigned to each particular frequency band. Guard bands, however, sacrifice useful bandwidth (thereby lowering total capacity) for dead frequency space used to reduce adjacent channel interference. In commercial communications systems (where capacity generates revenue), large guard bands are typically an undesirable alternative. Filters are also undesirable because they tend to cause signal distortion which can degrade performance and increase the complexity and expense of the communications system.
Another attempt at reducing co-channel and adjacent channel interference involves using lower frequency reuse factors. Lower frequency reuse factors reuse the same frequencies less often (geographically) and include, for example, seven-to-one and twenty-one-to-one frequency reuse factors. Lower frequency reuse factors increase the number of cells that may be used to separate other cells covering a common frequency band. Co-channel interference may therefore be reduced by increasing the physical distance between cells covering the same frequency band.
A necessary consequence of increasing the frequency reuse factor, however, is that the bandwidth of the individual spot beams is reduced. For example, in a twenty-one-to-one frequency reuse pattern, each spot beam has the bandwidth (and therefore capacity) of less than five percent (1/21) of the entire bandwidth available to the beam from which the spot beam was created. Below a certain level, the capacity of the spot beam can no longer accommodate the bandwidth requirements of the area that it is assigned to cover. As a result, a low reuse may not be an appropriate candidate for reducing interference. Furthermore, as the reuse is decreased, the antennas needed to generate the spot beams become more complicated and expensive.
A need has long existed in the industry for an improved method for reducing interference and increasing spectral efficiency.